The PreviewChanges recipe lets you see which lines changed before you save a page when you're editing. From the recipe's page:
While editing a page in "preview" mode, a diff block is shown before the rendered page output. The diff block contains, like a page history, highlighted differences between the saved version of the page and the currently edited text.
All you need to do is put the script in the cookbook/ directory.
You can do it from the command line like this:
#starting from the document root cd site/cookbook wget http://www.pmwiki.org/pmwiki/uploads/Cookbook/previewchanges.php
The pre-configured config.php configuration file will detect that the recipe file is installed and load it automatically. For other similar recipes, you would need to edit you config.php configuration script to cause the script to load.